If you make a mental note of something which requires special attention, such as the need to pick up a quart of milk on the way home, you make an effort to store it in your memory so that you will not forget it.

Typically someone makes a mental note when he or she is in haste, and does not have the time to write something down.

The implication is usually that something is reasonably important, but perhaps not crucial.

* She made a mental note to have his prescription refilled. 

* I made a mental note to call my mother and tell her what he'd said.

* Last time we had dinner together I made a mental note that you didn't like fish.